Preheat oven to 220ºC, (200ºC for fan assisted), 425ºF, Gas Mark 7. Have ready 2 baking sheets
Unroll pastry sheet and cut into 4 equal rectangles - prick with a fork, brush with beaten egg and scatter with poppy seeds. Bake for 8 minutes until risen and golden brown.
Meanwhile place fish fillets and fennel in ovenproof dish, drizzle with oil and then top with juice and zest from orange and season to taste.
Remove pastry from oven and keep warm, reduce temperature to 200ºC, (180ºC for fan assisted), 400ºF, Gas Mark 6. Put fish and fennel in oven and bake for approx. 20 minutes until fish is cooked.
Wilt the spinach in a pan with a little butter and oil. Slice pastry rectangles in half horizontally, place a base piece of pastry on a serving plate, top with a mixture of the fennel and spinach then one sea bass fillet and finally place the poppy seeded pastry on top.
Garnish with additional orange zest or slices if desired.